Although there ius no strong evidence that oral contraceptives (OCs) have an effect on breast cancer risk when used during the middle reproductive years, women who use OCs for long periods of time in the post-menarcheal period may be at increased risk of breast cancer. Previous studies that have examined this relationship disagree as to whether there is a significantly increased risk with early long-term OC use or no risk. Methodological differences and the premature timing of some of these studies may have contributed to the continued controversy. The LONG-TERM OBJECTIVE of this proposal is to conduct a case-control study of breast cancer among young women using a carefully planned protocol. The SPECIFIC AIMS are (1) to examine the hypothesis that early long-term OC use is associated with an increased risk of breast cancer and to determine whether any increased risk is modified by reproductive and other breast cancer risk factors, the particular formulation of OC used or temporal factors such as era of use, age at first use, time since first use and duration of use; (2) to establish the reliability of retrospective recall of OC use by comparing interview histories with medical records; and (3) to investigate the relationship between breast cancer and diagnostic and therapeutic radiation, abortion, regularity of menstrual cycling, physical activity, alcohol and caffeine consumption practices and smoking habits. METHODS: A matched case-control design will be used. Cases will be white, English speaking women aged 40 and under, diagnosed with breast cancer between January, 1985 and December, 1990, and identified by our population-based tumor registry. Controls will be individually matched to cases by age, race, parity (ever full-term pregnancy vs. never full-term pregnancy) and neighborhood of residence. In-person structured interviews will be conducted. To facilitate recall, a photograph album of all OCs ever sold in the United States and a comprehensive calendar to record life events will be used during the interview. The 720 case-control pairs that will be interviewed are sufficient to allow for the evaluation of OC and other effects on breast cancer risk in the presence of confounding variables and to test for interactions among risk factors.